Problem: The scale on a map is 4cm : 3km. If two cities are 28cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 4cm on the map corresponds to an actual distance of 3km. A distance of 28cm on the map is the same as 7 $\cdot$ 4cm. The actual distance between the two cities is 7 $\cdot$ 3km, or 21km.